GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 213-8
Presentation Time: 3:15 PM


PORTENGA, Eric W.1, BIERMAN, Paul R.2, TRODICK, Charles D.2, DEJONG, Benjamin3, ROOD, Dylan H.4 and PAVICH, Milan J.5, (1)Department of Geography & Geology, Eastern Michigan University, 140V Strong Hall, Ypsilanti, MI 48197, (2)Department of Geology, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, (3)185 Alpine Meadow Lane, 185 Alpine Meadow Lane, 12201 Sunrise Valley Drive MS 926A, Waterbury, VT 05676, (4)Department of Earth Science and Engineering, Imperial College London, Royal School of Mines, South Kensington Campus, London, SW7 2AZ, United Kingdom, (5)U.S. Geological Survey (emeritus), 12201 Sunrise Valley Drive, MS 926A, Reston, VA 20192

The ecological health of Chesapeake Bay deteriorated following colonial American settlement, over-fishing, and regional deforestation in the 18th–19th centuries. Modern efforts to restore the ecological health of Chesapeake Bay have, in part, called for a reduction of sediment flux from the Bay’s contributing tributaries, of which the Potomac River is the largest. Through this analysis of landscape dynamics within the Potomac River watershed, we find that modern sediment yields (n = 10) remain higher than pre-disturbance erosion rates derived from in-situ produced cosmogenic 10Be (10Bei; avg. = 29.6 ± 14.1 Mg km-2 yr-1; n = 62). Cosmogenically determined rates are consistent with model-based regulatory limits for the Total Maximum Daily Level of sediment flux from the Potomac River to Chesapeake Bay (24.3 Mg km-2 yr-1). Additionally, this study provides the opportunity to test recently-developed methods for deriving denudation rates from measurements of meteoric 10Be (10Bem) normalized to measurements of the reactive phase of naturally-occurring 9Be (9Bereac), which is weathered from parent bedrock minerals. In the Potomac River basin, we find that the average 10Bem/9Bereac denudation rate (avg. = 40.0 ± 21.7 Mg km-2 yr-1; n = 56) is greater, though within the uncertainty of the average 10Bei erosion rate. When compared basin-by-basin, however, 10Bem/9Bereac denudation rates do not replicate 10Bei erosion rates well. We interpret the disparity between 10Bei erosion rates and 10Bem/9Bereac denudation rates as reflecting lingering effects of post-colonial landscape disturbance. We suggest that our 10Bem samples do not contain the high concentrations of 10Bem associated with pre-disturbance topsoil, which was eroded following deforestation and is now likely stored behind crumbling mill dams or in the tidal reaches of the Potomac River downstream from Washington, DC. The possibility remains that had post-colonial landscape change not occurred in the Potomac River basin, 10Bem/9Bereac denudation rates would more consistently reflect 10Bei erosion rates.